Microstructure products, such as pressure sensors, can be produced via semiconductor wafer processing or MEMS processing, hereafter referred to as MEMS processing. The microstructure product can be fabricated on a device substrate using current and well known MEMS processing techniques. A further step that is often required is bonding the device substrate to a handler substrate. Furthermore, gold pads or gold interconnect schemes are often used because of gold's high reliability and low corrosion characteristics. Gold and silicon, however, have a eutectic melting point of about 377-385° C. and bonding operations often exceed that temperature.
Techniques known to those practiced in the art of wafer processing can be used to fabricate a device on a device substrate. MEMS processing is a stepwise process with one step following another. One of the last steps is contact formation.
FIG. 4, labeled as “prior art”, illustrates a device substrate 101 with a passivation layer 103 and a contact 102. The passivation layer 103 is often a layer of silicon dioxide, often simply called “oxide”. The passivation layer 103 can also be silicon nitride, often simply called “nitride”. The contact 102 is an electrical contact, which is a part of a microcircuit or MEMS device where electrical current or electrical voltage is applied. FIG. 4 does not show a MEMS device element or any microcircuit element other than the contact 102. Those practiced in the art of MEMS are familiar with the processing steps leading up to and including contact formation. Furthermore, they are aware of the vast variety of devices and circuits that are typically present on a device substrate that has a contact 102. The contact 102 can consist of platinum silicide (PtSi) as an example of a typical contact material.
FIG. 5, labeled as “prior art”, illustrates a device substrate 101 with a contact 102, passivation layer 103, and an adhesion layer 104. The structure illustrated in FIG. 5 can be produced from that of FIG. 4 by depositing a layer of material to form the adhesion layer. Those practiced in the art of MEMS know many ways to deposit a layer of material over a substrate. The adhesion layer 104 can consist of titanium tungsten (TiW) or a similar material.
FIG. 6, labeled as “prior art”, illustrates a device substrate 101 with a contact 102, passivation layer 103, adhesion layer 104, and gold layer 105. The structure illustrated in FIG. 6 can be produced from that of FIG. 5 by depositing a layer of gold over the adhesion layer.
FIG. 7, labeled as “prior art”, illustrates a device substrate 101 with a contact 102, passivation layer 103, adhesion layer 104, and gold layer 105 after patterning. The structure of FIG. 7 can be formed from that of FIG. 6 by patterning the adhesion layer 104 and the gold layer 105. The fact that the gold layer 105 does not completely cover the adhesion layer 104 is a standard part of MEMS processing that is not special, but should be noted.
Structures similar to that illustrated in FIG. 7 are produced on device substrates in order to take advantage of the properties of the gold layer. After patterning, however, the gold layer 103 remains over the contact 102. Pinhole defects in the adhesion layer 104 offer an opportunity for gold to diffuse into the underlying contact. If a pinhole or other defect in the adhesion layer 104 is present then the gold will be physically touching the underlying contact material 102.
The eutectic melting point of gold and silicon, approximately 385° C., is the temperature at which gold that is touching the underlying silicon contact will actually start to melt. If gold melts into this contact, then gold diffuses into the silicon device substrate, and a fabrication error occurs resulting in a manufacturing failure. As mentioned above, an adhesion layer, also called a barrier layer, between the gold layer and the underlying substrate or contact is normally used to prevent this from occurring, but these layers can have pinhole defects. Gold can diffuse through a pinhole defect into an underlying contact or into the substrate. Furthermore, this failure mode is avoided in more typical wafer processing by making sure that the wafer never rises above the gold-silicon eutectic temperature (i.e. approximately 377-385° C.)
In MEMS processing, a silicon (Si) wafer is commonly used as a device substrate. A handler substrate is frequently required for mechanical isolation or other reasons well known to those familiar with MEMS processing. This handler substrate is often a Si wafer or a glass wafer. A number of techniques can be used to bond the device substrate to the handler substrate. Three of those techniques are anodic bonding, glass frit bonding, and eutectic bonding.
In anodic bonding, the substrates can be bonded from approximately 300° C. up to nearly 500° C. by placing and clamping the substrates between two metal electrodes. A high direct current (DC) potential is applied between the electrodes creating an electrical field, which penetrates the substrates. If the handler substrate is a glass that contains sodium ions then at the elevated temperature the sodium ions are displaced from the bonding surface of the glass by the applied electrical field. The depletion of sodium ions near the surface of the glass makes the surface highly reactive with the silicon surface of the device substrate. The high reactivity results in a solid chemical bond between the two substrates.
In glass frit bonding, a viscous glass material is coated on one or both of the wafers to be bonded. This frit is sometimes heat treated to drive off solvents and binders. The wafers are then aligned if necessary and brought together. The wafers are then clamped under pressure and heated to temperatures that are typically in the range of 400° C. to 550° C. The glass frit flows and bonds to the two surfaces.
In eutectic bonding one substrate is coated with a first component of a two component eutectic bonding system and the other substrate is coated with the second component. The substrates are heated and brought into contact. Diffusion occurs at the interface and an alloy is formed. The eutectic composition alloy at the interface has a lower melting point than the materials on either side of it, and hence the melting is restricted to a thin layer. It is this melted eutectic layer that forms the bond.
All of these wafer bonding techniques use temperatures that are above the gold-silicon eutectic, but are not so high that the fabricated circuit would be ruined or destroyed. FIG. 8, labeled as “prior art” illustrates a device substrate 101 and a handler substrate 801 that are bonded together by a bond 802.
Aspects of the embodiments directly address the shortcomings of the prior art by patterning the gold layer 105 such that it does not overlay the contact 102.